Positive Electrode Active Substance For Lithium Ion Batteries, Positive Electrode For Lithium Ion Batteries, And Lithium Ion Battery

ABSTRACT

The present invention provides a positive electrode active material for lithium ion battery which has a high capacity and good rate characteristics can be provided. The positive electrode active material for lithium ion battery has a layer structure represented by the compositional formula: Li x (Ni y M 1-y )O z  (wherein M represents at least one type selected from the group consisting of Mn, Co, Al, Mg, Cr, Ti, Fe, Nb, Cu and Zr, x denotes a number from 0.9 to 1.2, y denotes a number from 0.5 to 0.65, and z denotes a number of 1.9 or more), wherein the coordinates of the lattice constant a and compositional ratio (Li/M) are within the region enclosed by three lines given by the equations: y=−20.86x+59.079, y=35x−99.393, and y=−32.946x+95.78 on a graph in which the x-axis represents a lattice constant a and the y-axis represents a compositional ratio (Li/M) of Li to M, and the lattice constant c is 14.2 to 14.25.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a positive electrode active material for lithium ion battery, a positive electrode for lithium ion battery, and a lithium ion battery.

2. Description of Related Art

A lithium-containing transition metal oxide is generally used as the positive electrode active material of a lithium ion battery. Specific examples of the lithium-containing transition metal oxide include lithium cobaltate (LiCoO₂), lithium nickelate (LiNiO₂), and lithium manganate (LiMn₂O₄). The complexation of these metal oxides is undergone to improve the characteristics (high capacity, cycle characteristics, preserving characteristics, reduction in internal resistance, and rate characteristics) and safety. The characteristics different from those required for lithium ion batteries in mobile telephones and personal computers are required for lithium ion batteries used in large-sized battery applications such as car applications and road leveling applications. Particularly, high capacity and rate characteristics are regarded as important.

Various methods have been used to attain high capacity and to improve the rate characteristics. For example, Patent document 1 discloses a lithium battery positive electrode made of a complex oxide represented by the general formula Li_(w)Ni_(x)Co_(y)Al_(z)O₂ (wherein w=0.90 to 1.10, x=0.80 to 0.95, y=0.04 to 0.19, z=0.01 to 0.16, and x+y+z=1.0) and also describes that a lithium battery positive electrode material, which has a large discharged capacity, is reduced in the deteriorations of battery characteristics caused by repetitive charge/discharge, is superior in cycle characteristics, is limited in the generation of gas caused by the decomposition of a positive electrode material after charged, and improved in preservability/safety, can be provided.

Also, Patent document 2 discloses a complex oxide represented by the general formula A_(w)D_(v)Ni_(x)Al_(y)N_(z)O₂ (wherein A represents at least one type selected from alkali metals, D represents at least one type selected from Mg and B, N represents at least one type selected from Si, Ca, Cu, P, In, Sn, Mo, Nb, Y, Bi, and Ga, w, v, x, y, and z respectively denote a number given by the following formulae: 0.05≦w≦1.2, 0.001≦v≦0.2, 0.5≦x≦0.9, 0.1<y≦0.5, and 0.001≦z≦0.2) as a positive electrode active material in a battery which comprises a negative electrode, a positive electrode, and a nonaqueous electrolyte including a lithium salt and can be plurally charged/discharged reversively. Patent document 2 also describes that this oxide enables a secondary battery positive electrode material to excel in all battery characteristics such as high capacitization, long life, rate characteristics, high-temperature characteristics, and safety.

-   (Patent document 1) Japanese Patent Application Publication No.     10-321224 -   (Patent document 2) Japanese Patent Application Publication No.     10-208744

SUMMARY OF INVENTION

However, high capacitization and rate characteristics are important characteristics required for a battery and there is a room for improvement of a high-quality positive electrode active material for lithium ion battery.

In view of this situation, an object of the present invention is to provide a positive electrode material for lithium ion battery having a high capacity and good rate characteristics.

The inventors have made earnest studies, and as a result, attracted their attentions to the relation between the lattice constant a of the positive electrode active material, compositional ratio of Li to metals (M) other than Li, and battery characteristics, to find that a battery produced using the positive electrode active material has good characteristics if the coordinates of a lattice constant a and compositional ratio (Li/M) are within a predetermined region on a graph in which the x-axis represents the lattice constant a and the y-axis represents the compositional ratio (Li/M) of Li to M.

According to a first aspect of the present invention completed based on the above teachings, there is provided a positive electrode active material for lithium ion battery which has a layer structure represented by the compositional formula: Li_(x)(Ni_(y)M_(1-y))O_(z) (wherein M represents at least one type selected from the group consisting of Mn, Co, Al, Mg, Cr, Ti, Fe, Nb, Cu and Zr, x denotes a number from 0.9 to 1.2, y denotes a number from 0.5 to 0.6 and z denotes a number of 1.9 or more), wherein the coordinates of the lattice constant a and compositional ratio (Li/M) are within the region enclosed by three lines given by the equations: y=−20.186x+59.079, y=35x−99.393, and y=−32.946x+95.78 on a graph in which the x-axis represents a lattice constant a and the y-axis represents a compositional ratio (Li/M) of Li to M.

In an embodiment of the positive electrode active material for lithium ion battery according to the present invention, the coordinates of the lattice constant a and compositional ratio (Li/M) are within the region enclosed by three lines given by the equations: y=−15x+44.18, y=33.33x−94.665, and y=−200x+575.92, and the lattice constant c is 14.2 to 14.25.

In another embodiment of the positive electrode active material for lithium ion battery according to the present invention, M is one metal selected from the group consisting of Ni, Mn, and Co.

According to another aspect of the present invention, there is provided a positive electrode for lithium ion battery comprising the positive electrode active material according to the present invention.

According to a further aspect of the present invention, there is provided a lithium ion battery comprising the positive electrode for lithium ion battery according to the present invention.

ADVANTAGEOUS EFFECT OF THE INVENTION

According to the present invention, a positive electrode active material for lithium ion battery which has a high capacity and good rate characteristics can be provided.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a graph of “lattice constant a”-“compositional ratio of Li/M” according to an example.

DETAILED DESCRIPTION OF EMBODIMENTS Structure of Positive Electrode Active Material for Lithium Ion Battery

As the material of the positive electrode active material for lithium ion battery according to the present invention, compounds useful as the positive electrode active material for the positive electrode of usual lithium ion batteries may be widely used. It is particularly preferable to use a lithium-containing transition metal oxide such as lithium cobaltate (LiCoO₂), lithium nickelate (LiNiO₂), and lithium manganate (LiMn₂O₄). The positive electrode active material for lithium ion battery which is produced using materials like the above has a layer structure represented by the compositional formula: Li_(x)(Ni_(y)M_(1-y))O_(z) (wherein M represents at least one type selected from Mn, Co, Al, Mg, Cr, Ti, Fe, Nb, Cu and Zr, x denotes a number from 0.9 to 1.2, y denotes a number from 0.5 to 0.6, and z denotes a number of 1.9 or more).

The ratio of lithium to the total metals in the positive electrode active material for lithium ion battery is 0.9 to 1.2. It is because a stable crystal structure is scarcely kept when the ratio is less than 0.9 whereas high capacity of the battery cannot be secured when the ratio exceeds 1.2.

The positive electrode active material for lithium ion battery according to the present invention has the characteristics that the coordinates of the lattice constant a and compositional ratio (Li/M) are within the region enclosed by three lines given by the equations: y=−20.186x+59.079, y=35x−99.393, and y=−32.946x+95.78 on a graph in which the x-axis represents the lattice constant a and the y-axis represents the compositional ratio (Li/M) of Li to M, and the lattice constant c is 14.2 to 14.25. When the lattice constant c is 14.2 to 14.25 and the coordinates of the lattice constant a and compositional ratio (Li/M) are within the above described region, the battery capacity using the positive electrode active material can be increased and the rate characteristics can be excellent.

Also, the coordinates of the lattice constant a and compositional ratio (Li/M) are preferably within a narrower region enclosed by three lines given by the equations: y=−15x+44.18, y=33.33x−94.665, and y=−200x+575.92, and the lattice constant c is preferably 14.22 to 14.25.

The positive electrode active material for lithium ion battery is constituted of primary particles, secondary particles formed from aggregated primary particles, or a mixture of primary particles and secondary particles. The average particle diameter of these primary and secondary particles of the positive electrode active material for lithium ion battery is preferably 2 to 8 μm.

When the average particle diameter is less than 2 μm, this makes it difficult to apply the positive electrode active material to the current collector. When the average particle diameter exceeds 8 μm, voids are easily produced when the active material particles are filled, leading to less fillability. The average particle diameter is more preferably 3 to 6 μm.

(Structure of Positive Electrode for Lithium Ion Battery and Lithium Ion Battery Using Positive Electrode)

The positive electrode for lithium ion battery according to an embodiment of the present invention has a structure in which a positive electrode mix prepared by blending, for example, a positive electrode active material for lithium ion battery which has the aforementioned structure, a conductive adjuvant, and a binder is applied to one or both surfaces of a current collector made of an aluminum foil or the like. Also, a lithium ion battery according to the embodiment of the present invention is provided with the positive electrode for lithium ion battery having such a structure.

(Method for Producing Positive Electrode Active Material for Lithium Ion Battery)

Next, a method for producing a positive electrode active material for lithium ion battery according to the embodiment of the present invention will be explained in detail.

First, a metal salt solution containing an oxidant is prepared. The metal salt is a sulfate, chloride, nitrate, acetate, or the like and, particularly, a nitrate is preferable. This is because the nitrate can be calcined as it is, so that a cleaning process can be omitted, even if the nitrate is mixed as impurities in the calcination raw material, and the nitrate functions as an oxidant to promote oxidation of metals in the calcination raw material. The metal contained in the metal salt is Ni and at least one or more types selected from Mn, Co, Al, Mg, Cr, Ti, Fe, Nb, Cu, and Zr. As the nitrate of a metal, for example, nickel nitrate, cobalt nitrate, or manganese nitrate may be used. At this time, the metal salt is prepared such that each metal is contained in a desired molar ratio. The molar ratio of each metal in the positive electrode active material is thereby determined.

Next, lithium carbonate is suspended in pure water, and then, a metal salt solution of the above metal is poured into the mixture to produce a lithium salt solution slurry. At this time, lithium-containing carbonate microparticles precipitate in the slurry. In this case, a sulfate or chloride is washed with a saturated lithium carbonate solution and then separated by filtration when the lithium compound does not react with the metal salt in the heat-treatment. When, like the case of using a nitrate or acetate, the lithium compound reacts as the lithium raw material during heat treatment, these metal salts are not washed and separated as it is by filtration, followed by drying, thereby enabling the salt to be used as a calcination precursor.

Next, the separated lithium-containing carbonate is dried to obtain a lithium salt composite (precursor of a positive electrode active material for lithium ion battery) powder.

Next, a sagger having a predetermined capacity is prepared and the powder of the precursor of a positive electrode active material for lithium ion battery is filled in the sagger. Next, the sagger filled with the powder of the precursor of the positive electrode active material for lithium ion battery is transferred to a kiln to calcine. The calcination is performed by keeping the sagger with heating for a predetermined time in an oxygen atmosphere. Also, it is desirable that the calcination is performed under a pressure of 101 to 202 KPa because the quantity of oxygen in the composition is increased. The calcination temperature is 700 to 1100° C., and the calcination is carried out preferably at 700 to 950° C. when y in the above formula satisfies the equation: 0<y≦0.5 and at 850 to 1100° C. when y in the above formula satisfies the equation: 0.5<y≦0.7. The crystallinity of the positive electrode active material is largely caused by the relation between the composition and calcination temperature. At this time, there is the case where even a small difference in composition affects the crystallinity of the positive electrode active material though depending on the range of calcination temperature. When the positive electrode active material precursor is made to have a proper compositional ratio and calcined at a proper calcination temperature corresponding to the compositional ratio, the crystallinity of the positive electrode active material is improved to make a high-performance positive electrode active material. Also, the crystallinity of the positive electrode active material is affected not only by the above factor but also by the grain size of the precursor and the amount of lithium carbonate used as the raw material. When the amount of lithium carbonate is large and a lot of lithium is contained in the positive electrode material precursor, the calcination proceeds smoothly. In this case, the lattice constant c is decreased with increase in calcination temperature whereas the lattice constant c is increased with decrease in calcination temperature because the calcination is insufficient.

After that, the powder is taken out of the sagger and ground to obtain a positive electrode active material powder.

In this case, when a nitrate is used as the metal salt to be poured in the production of the lithium salt solution slurry, a positive electrode active material containing oxygen exceeding that in the compositional formula is finally produced. Also, when the calcination of the positive electrode precursor is performed not under atmospheric pressure but under a predetermined pressure, a positive electrode active material containing oxygen exceeding that in the compositional formula is finally produced. When the positive electrode active material contains oxygen exceeding that in the compositional formula as mentioned above, a battery using the positive electrode active material is improved in various characteristics.

EXAMPLES

Although examples are provided for facilitating understanding of the present invention and its advantage, the present invention is not limited to the following examples.

Examples 1 to 29

First, lithium carbonate to be charged in an amount as described in Table 1 was suspended in 3.2 liter of pure water, and then, 4.8 liter of a metal salt solution was added to the mixture. Here, the metal salt solution was prepared in such a manner that the compositional ratio of a hydrate of a nitrate of each metal was that described in Table 1 and the number of moles of all metals was 14.

In this case, the amount of lithium carbonate to be suspended is a value at which x in the formula Li_(x)(Ni_(y)M_(1-y))O_(z) of a product (positive electrode for lithium ion secondary battery, that is, positive electrode active material) accords to that described in Table 1 and is calculated according to the following equation.

W(g)=73.9×14×(1+0.5X)×A

In the above formula, “A” is a value multiplied in order to subtract, in advance, the amount of lithium originated from a lithium compound other than lithium carbonate left in the raw material after filtration besides the amount required for the precipitation reaction. “A” is 0.9 when, like the case of using a nitrate or acetate, the lithium salt reacts as the calcination raw material, and 1.0 when, like the case of using a sulfate or chloride, the lithium salt does not react as the calcination raw material.

Though lithium-containing carbonate microparticles were precipitated in the solution by this treatment, this precipitate was separated by filtration using a filter press.

Subsequently, the precipitate was dried to obtain a lithium-containing carbonate (precursor of positive electrode active material for lithium ion battery).

Next, a sagger was prepared to fill the lithium-containing carbonate therein. Next, the sagger was placed in an oxygen ambient furnace under atmospheric pressure and heated to 800 to 940° C. for 4 hr. Then, the sagger was kept at this temperature under heating for 12 to 30 hr and then, allowed to cool for 3 hr to obtain an oxide. Then, the obtained oxide was pulverized to obtain a positive electrode active material powder for lithium ion battery.

Example 30

In Example 30, the same procedures as in Examples 1 to 29 were carried out except that each metal of the raw material was altered to the composition shown in Table 1, and a chloride was used as the metal salt to precipitate a lithium-containing carbonate, which was then washed with a saturated lithium carbonate solution, followed by filtration.

Example 31

In Example 31, the same procedures as in Examples 1 to 29 were carried out except that each metal of the raw material was altered to the composition shown in Table 1 and a sulfate was used as the metal salt to precipitate a lithium-containing carbonate, which was then washed with a saturated lithium carbonate solution, followed by filtration.

Example 32

In Example 32, the same procedures as in Examples 1 to 29 were carried out except that each metal of the raw material was altered to the composition shown in Table 1 and the calcination was performed not under atmospheric pressure but under a pressure of 120 KPa.

Comparative Examples 1 to 22

In Comparative Examples 1 to 22, the same procedures as in Examples 1 to 29 were carried out except that each metal in the raw material was altered to the composition shown in Table 1.

TABLE 1 Li₂CO₃ calcination suspension compositional ratio (%) of each metal in all metals except Li temperature solution(g) Ni Co Mn Ti Cr Fe Cu Al Sn Mg (° C.) Example 1 1397 55 25 20 900 2 1406 55 25 20 850 3 1397 60 15 25 800 4 1397 60 15 20 2.5 2.5 850 5 1397 60 15 20 2.5 2.5 850 6 1397 60 15 20 5 800 7 1397 60 15 20 5 800 8 1397 60 15 20 5 800 9 1397 60 15 20 5 800 10 1397 60 15 20 5 800 11 1397 60 15 20 5 820 12 1406 60 15 20 5 820 13 1415 60 15 20 5 820 14 1406 50 30 20 880 15 1397 50 30 20 860 16 1397 65 20 15 820 17 1397 65 25 5 2.5 2.5 850 18 1397 65 25 5 5 850 19 1397 65 25 5 5 850 20 1397 65 25 5 5 850 21 1397 65 25 5 5 850 22 1397 65 25 5 5 850 23 1397 65 25 5 5 850 24 1397 60 30 5 2.5 2.5 850 25 1397 60 30 5 5 850 26 1397 60 30 10 860 27 1406 60 30 10 860 28 1397 60 20 20 860 29 1397 60 20 15 5 880 30 1500 60 15 25 820 31 1500 60 15 25 800 32 1397 60 15 25 820 Compar- 1 1397 55 25 20 940 ative 2 1406 55 25 20 880 Example 3 1397 60 15 25 830 4 1406 60 15 25 860 5 1415 60 15 25 860 6 1397 60 15 20 2.5 2.5 820 7 1397 60 15 20 5 820 8 1397 60 15 20 5 820 9 1397 60 15 20 5 820 10 1397 60 15 20 5 820 11 1397 60 15 20 5 820 12 1397 60 15 20 5 840 13 1397 60 15 20 5 840 14 1397 50 30 20 900 15 1406 60 30 10 870 16 1397 65 25 5 2.5 2.5 870 17 1397 65 25 5 5 870 18 1397 65 25 5 5 870 19 1393 65 25 5 5 870 20 1393 65 25 5 5 870 21 1397 50 30 20 890 22 1397 60 15 25 820

(Evaluation)

The contents of Li, Ni, Mn, and Co in each positive electrode active material were measured by induction coupling plasma atomic emission spectrometry (ICP-AES) to calculate the compositional ratio (molar ratio) of each metal. Also, the crystal structure was confirmed to be a layer structure by X-ray diffraction.

Moreover, each positive electrode material was measured by powder XRD diffraction to find the lattice constant from the diffraction pattern. Also, among the measured factors, the lattice constant a was made to lie on the x-axis and the compositional ratio (Li/M) of Li to M (all metals excluding Li) found from MS analysis was made to lie on the y-axis to draw a graph as shown in FIG. 1.

The positive electrode material, a conductive material, and a binder were weighed in a ratio of 85:8:7. The positive electrode active material and the conductive material were mixed in a solution prepared by dissolving the binder in an organic solvent (N-methylpyrrolidone) into a slurry, which was then applied to the surface of an Al foil and pressed after dried to produce a positive electrode. In succession, a 2032-type coin cell for evaluation in which Li was used as the counter electrode was manufactured and an electrolytic solution prepared by dissolving 1M-LiPF₆ in EC-DMC (1:1) was used. Then, the coin cell impregnated with the electrolytic solution was used to calculate the battery capacity ratio of a battery capacity at 1 C to a battery capacity at 0.2 C to obtain the rate characteristics of the battery. These results are shown in Table 2.

TABLE 2 discharged rate lattice lattice lattice capacity(0.2 C) characteristics constant a constant c volume v Li/M (A · h/g) (%) Example 1 2.8721 14.2318 101.666 1.131 171 88 2 2.8725 14.2331 101.710 1.145 171 88 3 2.8735 14.2374 101.808 1.142 170 88 4 2.8736 14.2382 101.821 1.116 171 88 5 2.8743 14.2383 101.875 1.099 171 88 6 2.8741 14.2393 101.861 1.096 171 89 7 2.8733 14.2326 101.761 1.085 173 88 8 2.8739 14.2412 101.865 1.089 171 88 9 2.8733 14.2350 101.774 1.087 172 88 10 2.8741 14.2390 101.862 1.095 172 89 11 2.8739 14.2383 101.843 1.095 174 89 12 2.8733 14.2361 101.781 1.086 172 89 13 2.8742 14.2371 101.854 1.076 172 89 14 2.8735 14.2352 101.793 1.078 173 88 15 2.8731 14.2335 101.751 1.105 176 89 16 2.8755 14.2431 101.991 1.059 173 88 17 2.8741 14.2408 101.875 1.088 171 88 18 2.8734 14.2277 101.734 1.115 180 88 19 2.8730 14.2257 101.690 1.085 172 88 20 2.8727 14.2224 101.644 1.087 171 88 21 2.8734 14.2313 101.761 1.079 179 88 22 2.8730 14.2293 101.711 1.089 175 88 23 2.8742 14.2280 101.787 1.067 180 88 24 2.8717 14.2383 101.889 1.110 170 88 25 2.8761 14.2480 102.072 1.025 171 88 26 2.8755 14.2463 102.014 1.056 171 88 27 2.8758 14.2462 102.032 1.036 170 88 28 2.8749 14.2460 101.970 1.045 171 88 29 2.8749 14.2436 101.950 1.08 170 88 30 2.8728 14.2304 101.710 1.067 165 86 31 2.8755 14.2314 101.909 1.081 162 85 32 2.8736 14.2354 101.782 1.09 178 90 Compar- 1 2.8762 14.2480 102.072 1.064 173 87 ative 2 2.8724 14.2318 101.690 1.156 166 89 Example 3 2.8714 14.2205 101.540 1.079 175 87 4 2.8724 14.2318 101.690 1.156 166 89 5 2.8735 14.2408 101.833 1.069 167 86 6 2.8741 14.2373 101.852 1.127 167 85 7 2.8732 14.2386 101.792 1.163 163 82 8 2.8762 14.2526 102.096 1.000 160 83 9 2.8752 14.2562 102.131 1.022 163 85 10 2.8761 14.2441 102.039 1.016 170 87 11 2.8772 14.2500 102.161 1.008 167 87 12 2.8721 14.2326 101.677 1.086 172 88 13 2.8711 14.2216 101.527 1.108 171 86 14 2.8715 14.2259 101.617 1.122 169 87 15 2.8748 14.2402 101.908 1.105 173 87 16 2.8756 14.2362 101.875 1.068 171 86 17 2.8745 14.2326 101.771 1.040 171 86 18 2.8772 14.2500 102.161 1.028 167 87 19 2.8755 14.2402 101.970 1.094 171 87 20 2.8728 14.2304 101.710 1.067 165 83 21 2.8734 14.1915 101.598 1.056 161 83 22 2.8754 14.2521 101.995 1.061 163 82

When the resultant data of Table 2 is plotted on a graph of FIG. 1 and linear lines are drawn so as to enclose the plotted points at which good capacity and rate characteristics of the battery are exhibited, it is found that these points are included within the region enclosed by three lines: (1) y=−20.186x+59.079, (2) y=35x−99.393, and (3) y=−32.946x+95.78 in FIG. 1.

Generally, considerable time is required to evaluate battery characteristics when a positive electrode active material is used for a battery. However, according to the present invention, the characteristics of a battery provided with a positive electrode active material having a predetermined lattice constant c can be evaluated only by writing the above three lines on a graph in which the x-axis represents the lattice constant a and the y-axis represents the compositional ratio (Li/M) of Li to M to determine whether or not the coordinates of the lattice constant a and compositional ratio (Li/M) are within the region enclosed by the three lines. Therefore, the time required for the evaluation of a battery can be shortened, which improves the efficiency of battery production and reduces production cost.

Moreover, it is found that if lines are drawn so as to enclose those having more excellent battery capacity and rate characteristics, they are included within the region enclosed by three lines: (1) y=−15x+44.18, (2) y=33.33x−94.665, and (3) y=−200x+575.92 in FIG. 1.

Also, Examples 1 to 29 and 32 each use a nitrate as the metal salt to be poured and therefore, positive electrode active materials containing oxygen exceeding that in the compositional formula is finally produced. When comparing Examples 30 and 31 using a chloride and sulfate as the metal salt with those having the same condition other than the metal salt, the battery characteristics are more improved (for example, comparison of Example 3 with Examples 30 and 31).

Moreover, in Example 32 in which the positive electrode material precursor was calcined not under atmospheric pressure but under a predetermined pressure, a positive electrode active material containing oxygen exceeding that in the compositional formula is finally produced. Therefore, when comparing Example 32 with those having the same condition other than the pressure, Example 32 was more improved in battery characteristics (for example, comparison of Example 3 with Example 32).

Comparative Examples 1 to 20 are excluded from the region enclosed by three lines: (1) y=−20.186x+59.079, (2) y=35x−99.393, and (3) y=−32.946x+95.78 in FIG. 1 and had inferior battery characteristics. Also, Comparative Examples 21 and 22 had inferior battery characteristics because they had a lattice constant c out of the defined range from 14.2 to 14.25, though they were included within the region enclosed by three lines: (1) y=−20.186x+59.079, (2) y=35x−99.393, and (3) y=−32.946x+95.78 in FIG. 1. 

1. A positive electrode active material for lithium ion battery which has a layer structure represented by the compositional formula: Li_(x)(Ni_(y)M_(1-y))O_(z) (wherein M represents at least one type selected from the group consisting of Mn, Co, Al, Mg, Cr, Ti, Fe, Nb, Cu and Zr, x denotes a number from 0.9 to 1.2, y denotes a number from 0.5 to 0.65, and z denotes a number of 1.9 or more), wherein the coordinates of the lattice constant a and compositional ratio (Li/M) are within the region enclosed by three lines given by the equations: y=−20.186x+59.079, y=35x−99.393, and y=−32.946x+95.78 on a graph in which the x-axis represents a lattice constant a and the y-axis represents a compositional ratio (Li/M) of Li to M, and the lattice constant c is 14.2 to 14.25.
 2. The positive electrode active material for a lithium ion battery according to claim 1, wherein the coordinates of the lattice constant a and compositional ratio (Li/M) are within the region enclosed by three lines given by the equations: y=−15x+44.18, y=33.33x−94.665, and y=−200x+575.92.
 3. The positive electrode active material for a lithium ion battery according to claim 1, wherein M is one metal selected from the group consisting of Ni, Mn, and Co.
 4. A positive electrode for lithium ion battery comprising the positive electrode active material described in claim
 1. 5. A lithium ion battery comprising the positive electrode for lithium ion battery described in claim
 4. 6. A positive electrode for lithium ion battery comprising the positive electrode active material described in claim
 2. 7. A positive electrode for lithium ion battery comprising the positive electrode active material described in claim
 3. 8. A lithium ion battery comprising the positive electrode for lithium ion battery described in claim
 6. 9. A lithium ion battery comprising the positive electrode for lithium ion battery described in claim
 7. 